Soil nutrient availability, poplar growth and biomass production on degraded agricultural soil under fresh grass mulch

Poplar plantations have been recently established in southwestern upland areas of China as a source of future timber and to restore degraded lands. However, poor site conditions limit the growth and ecological function of poplar plantations. Soil resource availability is hypothesized to have a significant effect on growth performance of poplars when planted in the degraded upland areas. This study whether the application of fresh grass mulch improved the performance of poplar growth on degraded agricultural soils. Poplar plantations were established in 2004 by planting dormant, unrooted 1-year-old stem cuttings of the clone ‘Zhonglinmeihe’, a hybrid of Populus deltoides × P. nigra. Circular areas were mulched around each tree at four rates: 0.0 kg/m2, 2.5 kg/m2, 5.0 kg/m2 and 7.5 kg/m2 fresh grass (Imperata cylindrical var. major). These equated to about 0 (M0, bare soil), 10,000 (M1), 20,000 (M2) and 30,000 (M3) kg/ha since only about 40% of the total plot area was mulched. The effect of grass mulch on soil nutrient availability, poplar growth and biomass production were monitored over 3 years. Application of fresh grass mulch markedly affected soil nutrient contents. As expected, both in the growing and non-growing seasons, plots with mulch had higher total N, available N, available P and available K concentrations than control plots, and the nutrient concentrations increased with mulching quantity. Grass mulch significantly affected tree height and diameter at breast height (DBH) growth of poplar plantations. At the end of third growing season, total tree height increment in M1, M2 and M3 treatments was 22.2%, 93.8% and 95.1% higher than that in M0 plots, whereas DBH increment in mulched plots was 24.3%, 91.0% and 102.7% greater than in bare soil plots, respectively. Productivity was enhanced by the addition of fresh grass mulch. Overall, the benefit of mulch on productivity was enhanced with greater fresh grass mulch. Compared to M0, the productivity in M1, M2 and M3 treatments was increased by 19.0%, 107.9% and 178.1% at the end of third growing season, respectively. The results of this experiment emphasized that mulching may therefore be an important technique for establishment and management of poplar plantation on degraded agricultural soils in the region. Annual application of 20,000 kg/ha fresh grass for soil improvement in young poplar plantations can be recommended from this study.